1. Field of the Invention
The present invention is related to application of a photo-voltaic array and particularly to a method and a circuit for tracking the maximum power of the photo-voltaic array.
2. Brief Description of the Related Art
Nowadays, development of the renewable energy source such as the solar power has become a great trend for pursuing the green energy along with the petrochemistry energy exhausted. Hence, the solar power system has been widely applied to the home appliances, the communication system, the traffic light and illumination system.
Referring to FIG. 1, the application of solar power system generally is to convert the solar energy to the electric energy with a photo-voltaic array 11 and the electric energy is supplied to the load 13 with the converter 12 or the inverter. In order to improve the efficiency of the photo-voltaic array 11, the maximum power tracking circuit 15, which includes the maximum power tracking unit 151, the current control unit 152, the pulse width modulation unit 153, and the gate driving unit 154, is employed to control the photo-voltaic array 11 operating on the maximum power point (MPP).
Referring to FIG. 2, the photo-voltaic characteristic curve of the conventional photo-voltaic array is illustrated. It can be seen in FIG. 2 that the output power PPV of the photo-voltaic array increases along with increase of the output illumination in case of the different solar illuminations; the output power PPV of the photo-voltaic array is divided into zone A and zone B according to the maximum power point PMAX shown in FIG. 2 in case of the same solar illumination. When the photo-voltaic array is operated at zone A, the output power PPV of the photo-voltaic array increases along with increase of the output voltage VPV of the photo-voltaic array; when the photo-voltaic array is operated at zone B, the output power PPV of the photo-voltaic array increases along with decrease of the output voltage VPV of the photo-voltaic array.
Therefore, the conventional way to track the maximum power of the photo-voltaic array is a method of perturbation and observation, and FIG. 3 illustrates how the method of perturbation and observation is operated. The solid line shown in FIG. 3 represents the photo-voltaic characteristic curve with the identical illumination; the dash line above the solid line represents the power operation curve at the time of load increase; the dash line under the solid line represents the power operation curve at the time of load decrease.
When the photo-voltaic array is operated at zone B, it is supposed that the load power increases to C11 from D1 shown in FIG. 3 and the load power should be equal to the output power of the photo-voltaic array such that the actual work point parallel moves toward the solid line and operates at point D2. Similarly, when the load power continues increasing and moves toward C12 from D2 shown in FIG. 3, the working point of the photo-voltaic array parallel moves toward D3 from C12. And so on, when the load power continues to increase to point C1(m−1), the output power of the photo-voltaic array is equal to the load power, and the photo-voltaic array is operated at the maximum point DM.
Under this circumference, if the load is increased continuously to point C1m in FIG. 3, the operation for the photo-voltaic array is changed to zone A and the actual operation voltage should be the same as the output voltage of the photo-voltaic array, or the load end voltage is proportional to the output voltage VPV of the photo-voltaic array with a value of KV (KV is a transfer function of the output voltage of the converter with respect to the input voltage) at the time of the load power increasing to point C1m; according to the transfer relationship between the load end voltage and the output voltage VPV of the photo-voltaic array, point Dm+1 on the photo-voltaic characteristic curve can be found corresponding to C1m. In FIG. 3, it is supposed that KV is equal to 1 and the load end voltage is equal to the output voltage VPV of the photo-voltaic array such that the actual working point on the photo-voltaic array moves vertically toward the solid line and operates at point Dm+1. Similarly, when the load increases continuously, the actual working point of the photo-voltaic array moves to Dn from Dm gradually and finally moves to point O.
On the contrary, when the load power decreases to C2(n−1) from Dn on the photo-voltaic characteristic curve, it is supposed that KV is equal to 1 and the load end voltage is equal to the output voltage VPV of the photo-voltaic array such that the actual working point on the photo-voltaic array moves vertically toward the solid line and operates at point Dn−1. And so on, the working point of the photo-voltaic array moves toward the maximum power point Dm along with the load power decreasing, and when the load power continues decreasing, the working point of the photo-voltaic array moves toward D1 from Dm continuously.
Thus, the method of perturbation and observation applied by the maximum power tracking unit 151 shown in FIG. 1 is to detect changes of the output power PPV and the output voltage VPV before and after the operation of load increase/decrease and to determine if the photo-voltaic array is operated at zone A or zone B such that next operation being for load increase or decrease is decided for the working point of the photo-voltaic array moving toward the maximum power point DM and operating at the maximum power point DM finally or perturbing in the vicinity of the maximum power point DM.
Referring to FIG. 4, a flowchart of the conventional method of the perturbation and observation for tracking the photo-voltaic array maximum power is illustrated. Step 41 is performed to detect the output voltage VPV (Vn) and the output current IPV (In) of the photo-voltaic array and to figure out the output power PPV (Pn) based on the detected output voltage VPV (Vn) and the output current IPV (In); then, step 411 is performed to determine changes of the output power Pb, Pn of the photo-voltaic array between before and after the operation of load increase/decrease; when the output power of the photo-voltaic array after the operation of load increase/decrease is greater, step 412 is performed to determine changes of the output voltage Vb, Vn of the photo-voltaic array between before and after the operation of load increase/decrease; when the output voltage of the photo-voltaic array after the operation of load increase/decrease is greater, it means that the photo-voltaic array is operated at zone A and step 413 should be performed to process the operation of load decrease via setting the operation command D=1, and when the output voltage of the photo-voltaic array after the operation of load increase/decrease is less, it means that the photo-voltaic array is operated at zone B and step 414 should be performed to process the operation of load increase via setting the operation command D=0.
On the contrary, in case of the output power after the operation of load increase/decrease being determined not greater in step 411, step 421 is performed to determine if the output power after the operation of load increase/decrease is less or keeps unchanging. If it is less, step 422 is performed to determine the changes of the output power Vb, Vn of the photo-voltaic array between before and after the operation of load increase/decrease; when the output voltage of the photo-voltaic array after the operation of load increase/decrease is less, it means that the photo-voltaic array is operated at zone A and step 424 should be performed to process the operation of load decrease via setting the operation command D=1, and when the output voltage of the photo-voltaic array after the operation of load increase/decrease is greater, it means that the photo-voltaic array is operated at zone B and step 423 should be performed to process the operation of load increase via setting the operation command D=0.
If the output power of the photo-voltaic array keeps unchanging in step 421, step 431 is performed to determine the changes of the output power Vb, Vn of the photo-voltaic array between before and after the operation of load increase/decrease; if there is no change, it means the photo-voltaic array has been operated at the maximum point and no operation of load increase/decrease is required. Afterwards, step 45 is performed respectively to renew the output power Pn and the output voltage Vn of the photo-voltaic array after the operation of load increase/decrease as the output power Pb and the output voltage Vb of the photo-voltaic array before the operation of load increase/decrease for the next round.
When the current control unit 152 in FIG. 1 receives the operation command of the maximum power tracking unit 151, and when the voltage V0 of the load 13 is constant, a target load current IREF of the operation of load increase/decrease can be figured out based on the detected load current I0 and the preset current difference ΔI so as to adjust cycling period signal TC and duty period signal DC of the pulse width modulation for the pulse width modulation unit 153 capable of generating required pulse width modulation signals GD1˜GDn and the gate driving unit capable of generating gate driving signals G1˜Gn to drive the converter 12 such that the load current I0 can be controlled to increase or decrease to the target load current IREF gradually as shown in FIG. 5.
It can be understood from the preceding description that the conventional maximum power tracking circuit 15 is employed to control the photo-voltaic array to operate at the maximum power point and the maximum power tracking unit 151 has to perform multiplication with the multiplier or accumulation with the adder to estimate the output power PPV of the photo-voltaic array, and it is necessary to adopt the register to store the voltage and the power of the previous round such that it not only results in hardware with complicated circuits but also is hard to be joined to the conventional pulse width modulation unit 153 for being constructed an integrated circuit chip with a function of tracking maximum power.